That's saying it's *not* known *only* to the sender and receiver,
because the assymetric cryptography is weak. If a MAC key to be shared
between you and me is derived by me shouting the key value to you in
the midst of a crowd, it's not very good either. -bsy
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Bennet S. Yee Phone: +1 619 534 4614 Email: bsy@cs.ucsd.edu
Web: http://www-cse.ucsd.edu/users/bsy/
USPS: Dept of Comp Sci and Eng, 0114, UC San Diego, La Jolla, CA 92093-0114